Method for electrophotographically producing a multicolor picture



Oct. 23, 1962 H. G. GREIG 3,060,021

METHOD FOR ELECTROPHOTOGRAPHICALLY PRODUCING A MULTICOLOR PICTURE Original Filed March 20, 1958 MII INVENOR. HAnoLn G. Gram@ M. im

nite rates 37,060,021 Patented Oct. 23, 1962 hcc 3,060,021 METHOD FOR ELECTRPHOTOGRAPHICALLY PRODUCNG A MULTICOLOR PICTURE Harold G. Greig, Princeton, N J., assigner to Radio Corporation of America, a corporation of Delaware Original application Mar. 20, 1953, Ser. No. 722,670. Di-

vided and this application Nov. 2, 1959, Ser. No.

This application is a division of my copending application Serial No. 722,670, led March 20, 1958.

This invention relates to electrostatic printing and particularly, but not exclusively, to improved electroscopic developer powders for electrostatic printing and to nnproved methods of electrostatic printing utilizing the 1mproved developer powders.

An electrostatic printing process is that type of process for providing a visible record, reproduction or copy which includes an an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an electrically-insulating layer. The process usually includes the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original, except that it may be a different size.

A typical electrostatic printing process may include producing an over-all electrostatic charge on the surface of a photoconductive insulating material such as selenium, anthracene, or zinc oxide dispersed in an insulating binder. A light image is focused on the charged surface, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition, to thus form an electrostatic image. The electrostatic image is rendered visible by applying a developer powder which is held electrostatically to the charged areas of the surface. The powder image thus formed may `be fixed directly to the photoconductive material or it may be transferred to another surface upon which the reproduced image may be desired and then fixed thereon. The fixing step commonly comprises fusing the developer powder to the photoconductive material by the application thereto of heat.

According to prior processes, reproduction of images in a plurality of colors could be accomplished by successively transferring powder images of different colored powders from the photoconductive surface to another surface. Briefly, this type of process comprises exposing a photoconductive plate, lirst, to an original through light filters which enable one color to be recorded, .and then developing with colored powder to produce a copy of that color, then repeating for each other color and sequentially transferring the powder images onto the same copy sheet.

Production of plural color images by processes which require the transferring of each individual color image to a copy sheet introduces problems which are extremely difficult to overcome. Registration of the separate images is probably the greatest of these problems. When attempting to lay down separate color images in contiguous areas elaborate precautions are necessary to insure that one image will not overlap another upon .a copy sheet. Other problems include: (a) loss of image detail and definition during transfer, (b) the surface of the photoconductive material must be cleaned after each image transfer, and (c) the photoconductive material is usually coated on one surface of a rigid plate making it diicult to transfer the powder image to nonflexible surfaces.

One object of this invention is to provide improved developer powders which facilitate electrostatic printing in a plurality of colors.

Another object of the invention is to provide improved developer powders for electrostatic printing and improved methods of electrostatic printing utilizing such developer powders.

Another object is to provide improved developer powders which make possible the electrostatic printing of plural color images in situ such that the separate colors occupy contiguous areas on an insulating surface.

A further object is to provide improved electroscopic developer powders and methods of electrostatic printing which obviate the need for any transfer steps in electrostatically producing plural color images.

In general, the foregoing objects and other advantages may be accomplished in accordance with the instant invention which provides an improved electroscopic developer powder consisting essentially of tinely divided particles of a semiconductive zinc oxide coated with a film-forming material. The hlm-forming material is electroscopic, has a melting point substantially within a range of from C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature just above its melting point. The zinc oxide particles constitute substantially from 50% to 85% by weight of the developer powder. When desired, a suitable coloring agent may be incorporated in the coating.

The zinc oxide particles contemplated in this invention have the property of being unable to hold an electrostatic charge in the dark for a period long enough to permit development of an electrostatic image. When a developer powder, made in accordance with this invention, is xed by fusing, it has a volume resistivity not greater than 1012 ohm-cm. and is incapable of being electrostatically overprinted. Thus, a rst powder image can be laid down on an insulating surface such as, for example, certain photoconductive surfaces, .and a second image of a different color can be laid down in those areas of the photoconductive insulating surface not covered by the irst image. When suitable coloring agents are incorporated, coiored developer powders are provided by means of which a composite color image can be produced wherein the different color powders are laid downv side by side in discrete non-overlapping areas.

in accordance with this invention, improved electrostatic printing methods are provided utilizing the above described electroscopic developer powders. One such method comprises the steps of (l) developing a latent electrostatic image on an insulating surface by applying thereto an electroscopic developer powder of the type described heretofore; (2) applying heat to said developer powder to cause the coating on the zinc oxide particles to melt and ow toward the insulating surface thereby causing portions of the zinc oxide particles to protrude above the coating material and leaving thereon only a very thin lm of coating material; (3) producing a second electrostatic image on the insulating surface having thereon the first developed image but not including any of the area covered by the first developed image; (4) applying a dierent colored electroscopic developer powder to the second electrostatic image to produce a second powder image in areas not covered by the iirst developed image; and (5) fixing the second powder image. When desired, the steps of the above method may be repeated to produce a composite color image in as many colors as desired. y

Other objects and advantages of this invention are more fully described in the following detailed description when read in conjunction with the accompanying drawings wherein:

yFIG. 1 is a partially-schematic sectional View of an apparatus for producing a blanket electrostatic charge upon an insulating surface.

FIG. 2 is a partially-sectional elevational view of an apparatus for projecting light to form by contact an electrostatic image upon the insulating surface of FIG. 1.

FIG. 3 is a sectional view of an apparatus for applying electroscopic developer powder, in accordance with this invention, Lto the image produced in FIG. 2.

FIG. 4 is a partially-schematic sectional view of an apparatus for fixing the developed image, produced in FIG. 3, to the insulating surface.

FIG. 5 is a partially-schematic sectional view illustrating the result obtained with the apparatus of FIG. 3 in accordance with the method of this invention.

Similar reference characters are applied to similar elements throughout the drawings.

SEMICONDUCTIVE ZINC OXIDE The selection of a Zinc oxide having suitable semiconductive properties is an important feature of this invention. Methods have been devised to select those which will function properly. Such methods include the following:

Method 1.--A mixture is prepared comprising about milligrams of dry zinc oxide powder and a few drops of an 80% solution of silicone resin in xylene (G.E.-SR 82, marketed by the General Electric Company, Silicone Products Division, Waterford, N.Y.) diluted with toluene in the ratio of 60 grams solution to 105 grams toluene. The mixture is coated on filter paper and dried to produce a dry coating over an area about 0.25 inch in diameter. The dry coating is cooled to about 190 C. and examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides which are suitable exhibit a green or yellow luminescence. Other zinc oxides produce a lavender or orange luminescence.

Method 2.-About 0.25 gram of dry zinc oxide powder is placed in a silica boat. The boat is inserted into a silica tube and the system flushed with hydrogen gas. The tube and boat are fired for about 5 minutes at about 1000 C. in a stagnant hydrogen atmosphere. The boat is cooled in hydrogen to room temperature. The red zinc'oxide is examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides which are suitable luminesce weakly or not at all. Other zinc oxides luminesce brightly.

As a result of the foregoing selection methods, it has been found that the class of zinc oxides known as American process zinc oxides have suitable semiconductive properties. When zinc oxides of this class are combined with a suitable coating material as taught herein, the resultant electroscopic developer powder, when fused, has a volume resistivity of 1012 ohm-cm. or less. Superior results are obtainable -by selecting a zinc oxide which exhibits a green color in Method 1 and which does not luminesce at all in Method 2.

COATING MATERIALS of a suitable coating material for the zinc oxide particles is an important feature of this invention. A material is normally selected having a melting point less than the temperature at which paper will char. A preferred temperature range is between 90 C. and 250 C. It is also important that the coating be one which, when applied to an insulating surface and fused thereon, will function as a binder holding the zinc oxide on the insulating surface. The viscosity of the coating material comprises another important criterion. The viscosity must be low enough so that when melted the coating will ow olf the zinc oxide particles leaving them partially exposed or protruding with only a thin lm of coating material on the protruding portions. After the coating material has been melted and fused to an nsulating surface it is essential that the protruding particles of zinc oxide shall present a matte surface i.e. the lm of coating material remaining on the protruding portions of zinc oxide must not be thick enough to provide Proper selection 4 a gloss finish. Were the hlm of coating to have such a thickness, the particles of zinc oxide might be insulated to a degree as to cause the coating material to retain an electrostatic charge and thereby impair subsequent printing operations. It is extremely` diicult to measure the thickness of such a film, however, if a physical appearance substantially like that described is achieved, the developer powder will have the characteristics contemplated in this invention. It is preferred that the coating should not be so free flowing as to allow it to migrate into unwanted areas of the insulating surface when melted. A preferred viscosity range is from 45 cps. to 10,000 cps. as measured with a direct reading Brookfield viscosimeter with a spindle speed of 60 r.p.m. at a temperature just slightly above the melting point of the material. Finally, it is essential that the coating on the zinc oxide particles have electroscopic properties so that the coated particles may -be electrostatically attracted to charged areas of the insulating surface.

Coating materials having the foregoing properties may comprise certain natural or synthetic resins, waxes or other low melting materials or mixtures thereof. For example, any of the following materials or combinations of materials may be used:

(l) Carnauba wax (2) Polymekon Wax (a chemically modified microcrystalline wax of the Warwick Wax Co., New York, N.Y.).

(3) Ultracera Amber Wax (a microcrystalline petroleum wax of the Bareco Oil Co., Barnsdall, Oklahoma).

(4) BE Square Wax White (a microcrystalline petroleum wax of the Bareco Oil Co.).

(5) Petronauba D Wax (a microcrystalline petroleum wax of the Bareco Oil Co.).

(6) Piccolyte S135 (a thermoplastic hydrocarbon terpene resin of the Pennsylvania Industrial Chemical Corp., Clairton, Pennsylvania).

(7) A mixture of Polymekon Wax and Piccolyte S- 115.

(8) A mixture of Acrawax C (a synthetic wax-octadecenamide, of the Glyco Products Co., Brooklyn, New York) and calcium stearate.

(9) A mixture of Acrawax resin.

Coating material such as those specified may also include modifying agents such as plasticizers, toughening agents, hardening agents, dispersing agents, etc. which are added to obtain desired physical and electrical properties.

A developer powder of this invention includes a ratio of zinc oxide particles to coating material within a range of from 1 to 7 parts by weight of zinc oxide particles to l part by weight of coating material. Such a powder is generally prepared by first melting the coating material and then dispersing finely divided zinc oxide particles in the melt. The melt is allowed to cool and harden after which it is broken up and reduced to the desired powder form. The ratio of zinc oxide to coating material speciiied above is important in a given developer powder formula. The exact ratio depends to a large extent on the particle size and the dispersion of the zinc oxide chosen.

C and a solid silicone COLORING AGENTS Coloring agents such as dyes, stains or pigments can be added lto the melt to produce developer powders of a desired color. Examples of suitable coloring agents include:

(1) Cyan Blue Toner GT (described in U.S. Patent 2,486,351 to Richard H. Wisw-all, IL).

(2) Benzidine Yellow.

(3) Brilliant Oil Blue BMA (Color Index No. CJ. 61555, National Aniline Division of Allied Chemical and Dye Corp., New York, N.Y.).

(4) Sudan III Red (Color Index No. 26100, Fisher Scientific Co., Pittsburgh, Pennsylvania).

Oil Yellow 2 G (Color Index No. 11020, American Cyanamid, New York, N.Y.).

(6) Oil Red N-1700 (Color Index can Cyanamid, New York).

These and other suitable coloring agents may be employed singly or in combination to impart to the developer powder a desired color.

PRINTING PROCESSES The improved methods in accordance with this invention will now be described with reference to the drawings.

FIG. 1 illustrates a means for applying a uniform electrostatic charge to `an insulating surface 11. Usually, in electrostatic printing, the insulating surface 11 will comprise a photoconductive insulating coating on a substrate 12 which is illustrated herein as a sheet of paper. The photoconductive insulating coating may comprise selenium anthracene or a zinc oxide coating such as is described in Electrofax-Direct Electrophotographic Printing on Paper, by C. I. Young and H. G. Greig, RCA Review, volume 15, No. 4. The sheet 12 is positioned on a grounded metal plate 13 following which a corona charging -14 unit is passed one or more times over the insulating surface to provide thereon a uniform electrostatic charge.

The next step in the process, illustrated in FIG. 2, is to produce an electrostatic image on the photoconductive insulating surface 11. This may be accomplished by placing a photographic transparency 21 upon the charged photoconductive insulating surface 11 and exposing to light derived, for example, from a lamp 22 in the manner of conventional contact printing. Wherever the light strikes the surface 11, the electrostatic charge thereon is reduced or removed. This leaves `an electrostatic image or pattern of charges corresponding to the non-illuminated areas of the light image.

The electrostatic image may be stored for a while if desired. Ordinarily, the next step is to apply developer powder to the surface 11. Referring to FIG. 3, this may be accomplished by passing a developer brush 31 containing the developer powder across the surface 11 bearing the electrostatic image. Coated particles 32 of developer powder are deposited on those areas of the surface 11 retaining the electrostatic charge. The developer brush comprises a mixture of magnetic carrier particles, for example powdered iron, and the developer powder. rThe mixture is secured in a magnetic field by a magnet 33 to form the developer brush 31.

liethods of charging, `exposing `and developing may be employed other than those described with reference to FIGS. l, 2, and 3. For example, charging may be accomplished by friction, exposure by projection, and development by dusting the developer powder onto the insulating surface all as well known in the art.

For more detailed description of the corona charging method of FIG. l and the magnetic brush development of EEG. 3 reference is made to Electrofax-Direct Electrophotographic Printing on Paper by C. J. Young and H. G. Greig, RCA Review, volume 15, No. 4. Also described in this publication is a method of development called cascading. Cascading utilizes gravity to convey the developer powder, mixed with a carrier such as glass beads, across the insulating surface. This method of development is also contemplated in this invention.

The developed image is now fixed to the surface. This is easily accomplished, as shown in FIG. 4, by passing a resistance heating unit 41 over the image-bearing photoconductive insulating surface 11. When -a temperature above the melting point of the coating on the zinc oxide particles is -applied thereto, the coating melts and becomes bonded to the surface 11. Other means are available for fusing the developed image. For example, the heating element 41 may comprise -an infrared lamp, or the sheet 12 may be placed in an oven.

It is during the fixing step that one of the unique fea- No. 26120, Ameri- 6 tures of this invention becomes apparent. materials for the zinc oxide particles are selected as taught herein, a surface is obtained in developed areas as shown in FIG. 4. The coating material melts to form a continuous layer 42 adhering to the surface 11. In forming this continuous layer 42 the coating material melts off at least the topmost zinc oxide particles 43 leaving these portions protruding above the layer and covered only with a very thin filmt of coating material and forming a matte surface on the developed areas. Were the coating material to have too high a viscosity, for example greater than 10,000 cps., it is very unlikely that this result could be achieved. Such coating material would tend to adhere to the entire surface of the particles of zinc oxide even when heat is applied. Were the coating material to have too low a viscosity, for example less than 45 cps., there would be a tendency for it to spread into non-image areas, thereby causing 4a loss .in definition of the image. Because of this unique characteristic of the coating material a novel electrostatic printing procedure process is possible as will be described hereinafter.

NON-OVERPRINTING PROCEDURE This procedure involves the printing of a second powder image on a photoconductive insulating surface in areas not covered by -a first-deposited powder image. This is made possible by employing in the development step, described in connection with PIG. 3, the developer powders of this invention. Upon completion of the fixing step of FIG. 4, the fused developer powder of the first image provides a surface which is incapable of retaining an electrostatic charge. Accordingly, subsequent to the fixing step of FIG. 4, this procedure comprises the steps of (l) recharging the photoconductive insulating surface 1-1 with an over-all charge 'as shown in FIG. 1, the charge not being retained on those portions of the surface covered with the first fixed powder image, (2) exposing the surface to another light image as shown in PIG. 2 to produce a second electrostatic charge pattern or image on those areas not subjected to light, and (3) developing the second electrostatic image with a different colored developer powder. 'The results of this procedure are shown in FIG. 5. As shown therein the developer powder first deposited consists of particles of zinc oxide 43 and the fused coating material 42. Coated zinc oxide particles 32 are deposited in configuration with the second electrostatic image. Because the zinc oxide particles 43 in the first developer powder have the semiconductor pro erties specified herein, development of the second electrostatic image occurs only in charged Iareas on the photoconductive surface 11 not covered by the first deposited developer powder.

Charging, exposing, developing and fixing can be carried out a third time to produce a third powder image in areas on the photoconductive surface 11 contiguous to those occupied by the irst and second powder images.4 The last development step does not require the use of semiconductive particles in the developer powder such as described herein but, instead, may be accomplished with any type of developer powder commonly employed in the art of electrostatic printing.

In accordance with the methods of this invention electrostatic printing 4in a plurality of colors is made possible. For example: light may be projected through a color transparency and thence through filters capable of transmitting all colors except yellow. The electrostatic image so produced on a photoconductive surface is then developed with yellow colored developer powder. A second exposure is made through filters transmitting all coiors except blue and the development carried out with blue colored developer powder. In the same way, red light is filtered out and red developer powder appplied to the photoconductive surface. Thus in a three stage process, yellow, blue and red powder images are produced in separate contiguous areas on the photoconductive surface to provide a composite color image.

When coating DEVELOPER POwDERs WHITE DEVELOPER POWDER Parts by weight Carnauba wax l Semiconductive American process Zinc oxide 2 This is the simplest type of nOn-overprinting developer powder. The wax is melted and particles of the zinc oxide having a particle size from 0.025 to 0.5 micron mean diameter are added to lthe melt. Continuous stirring from to 30 minutes is sutiicient to thoroughly disperse the zinc oxide in the wax when the batch weighs about 100 grams. The mixture is then allowed to cool and harden after whichVi-t is reduced to a line powder. This is accomplished by ball milling the mixture for about 3 hours and then classifying it as to particle size. For most purposes, the fraction below 200 mesh (74 microns) is suitable for use as an electroscopic developer powder.

Example Il WHITE DEVELOPER POWDER Par-ts by weight Polymekon wax 15 Piccolyte S-l15 Semiconductive American 5 process zinc oxide 50 Prepared as in Example I except that the Polymekon wax and Piccolyte are melted together before adding `the zinc oxide.

Example Ill WHITE DEVELOPER POW'DER Parts by Weight Ultracera wax Semiconductive American Process zinc oxide Prepared as in Example I.

Example IV WHITE DEVELOPER POWDER Parts by weight BE square wax amber 20 Semiconductive American process Zinc oxide 25 Prepared as in Example I.

Example VI BLUE DEVELOPER POWDER Parts by weight Polymekon wax 15 Piccolyte S1l5 5 Semiconductive American process Zinc oxide 50 Condensation blue 1 Prepared as in Example II except coloring agent is added after the zinc oxide.

Example VII BLUE DEVELOPER POWDER Parts by weight Piccolyte S-135 20 S Parts by weight Semiconductive American process Zinc oxide 30 Cyan blue toner G.T 1.5

Piccolyte S-l35 20 Semiconductive American process zinc oxide 30 Brilliant oil blue B.M.A 1.0

Preparation as in Example VI.

Example IX YELLOW DEVELOPER POWDER Parts by weight Piccolyte S-l35 20 Semiconductive American process zinc oxide 30 Benzidine yellow 1.5

Preparation as in Example VI.

Example X GREEN DEVELOPER POWDER Parts by Weight GREEN DEVELOPER POWDER Parts by weight Piccolyte S-135 20 Semiconductive American process zinc oxide 30 Cyan blue G.T 1.0 Benzidine yellow 1.0

Preparation as in Example VI.

Example XII RED DEVELOPER POWDER Par-ts by weight Piccolyte S-135 20 Semiconductive American process zinc oxide 30 Oil red N1700 3.0 Oil yellow 2 G 1.2

Preparation as in Example VI.

rhere have been described new and improved electroscopic developer powders and methods of electrostatic printing which make possible electrostatic printing in a plurality of colors.

What is claimed is:

1. The method of electrostatic printing comprising the steps of: electrophotographically producing a rst electrostatic image On a photoconductive insulating surface; applying to said electrostatic image a rst fusible developer powder having a characteristic color and consisting essentially of particles of semiconductive zinc oxide having a coating thereon of a thermoplastic, electroscopic material having a melting point substantially within the range of from C. to 250 C. and a viscosity not in excess of 10,000 centipoises at a temperature slightly above the melting point of said material, said developer powder, when fused, having a bulk resistivity of up to l012 Ohmcm.; applying heat to said developer powder on said insulating surface to cause said coating to melt and flow toward said insulating surface leaving said particles of zinc oxide protruding from said coating material, and leaving only a thin ilrn of coating material on the portions of said particles protruding from said coating material to provide developed areas on said surface which are incapable of retaining electrostatic charge; electro- U photographically producing a second electrostatic image on said insulating surface bearing said iirst developer powder; and applying to said second electrostatic image a second electroscopic developer powder having a characteristic color different from that of said iirst developer powder whereby said developer powder is prevented from depositing on said insulating surface in areas covered by said first developer powder.

2. 'I'he method of electrostatic printing comprising the steps of: (l) producing a -rst electrostatic image on a photoconductive insulating surface; (2) applying to said electrostatic image a iirst fusible developer powder having a characteristic color and consisting essentially of particles of semiconductive Zinc oxide having a coating thereon of a thermoplastic, electroscopic material having a melting point substantially within a range of from 90 C. to 250 C. and a viscosity substantially within a range of from 45 to 10,000 centipoises at a temperature slightly above said melting point, said developer powder when fused having a bull: resistivity of up to 1012 ohm-cm.; (3) applying heat to said first developer powder on said insulating surface to cause said coating to melt and flow toward said insulating surface leaving particles of sa-id Zinc oxide protruding from said coating material and leaving only a thin film of coating material on the portions of said particles protruding from said coating material to provide developed areas on said surface which are incapable of retaining electrostatic charge; (4) repeating the procedures of steps 1), (2) and (3) employing therein a second fusible developer powder substantially the same as said iirst fusible developer powder but having a characteristic color diiering therefrom whereby said second developer powder is prevented from depositing on said insulating surface in areas covered by said first developer powder; (5) repeating the procedures of steps (l) and (2) employing therein an electroscopic developer powder having a characteristic color differing from either said first or said second fusible developer powders whereby said third developer powder is prevented from depositing on said insulating surface in areas covered by either of said iirst or second developer powders; and (6) fixing said electroscopic developer powder to said insulating surface.

3. The method of electrostatic printing comprising the steps of: producing a substantially uniform electrostatic charge upon a photoconductive insulating surface exposing said photoconductive surface to a light image to produce thereon a rst electrostatic image; applying to said first electrostatic image a fusible developer powder having a characteristic color and consisting essentially of par- 5 ticles of semiconductive zinc oxide having a coating thereon of a thermoplastic, electroscopic material having a melting point substantially within the range of from 90 C. to 250 C. and a Viscosity substantially within the 'range of from 45 to 10,000 centipoises at a temperature slightly above the melting point of said material, said developer powder when fused having a bulk resistivity of up to 1012 ohm-cm.; applying heat to said `fusible developer powder on said photoconductive surface to cause said coating to melt and flow toward said surface leaving particles of said Zinc oxide protruding from said coating material and leaving only a thin lilm of coating material on the portions of said particles protruding from said coating material to provide developed areas on said surface which are incapable of retaining electrostatic charge; again producing a substantially uniform electrostatic charge upon the exposed portions of said photoconductive insulating surface; exposing said surface with said rst developer powder thereon to a second light image to produce a second electrostatic image in areas on said surface not covered by said iirst developer powder, and applying a second developer powder having a characteristic color different from said 4iirst developer powder to said second electrostatic image whereby said second developer powder is prevented from depositing on said insulating surface in areas covered by said first developer powder.

`4. The method of electrostatic printing comprising the the steps of: (1) producing a substantially uniform electrostatic charge upon a photoconductive insulating surface; (2) exposing said photoconductive insulating surface to a first light image to produce thereon a first electrostatic image; (3) developing said first electrostatic image by applying thereto a fusible yellow developer powder consisting essentially of particles of semiconductive zinc oxide having a coating thereon of a thermoplastic, electroscopic material including a minor proportion of a yellow coloring agent, said material having a melting point substantially within a range of lfrom C. to 250 C. and a viscosity substantially within a range of from 45 to `10,000 centipoises at a temperature slightly above said melting point, said developer powder, when fused, having a volume resistivity of up to 1012 ohm-cm.; (4) fixing said developer powder to said photoconductive insulating surface by applying heat thereto to cause said coating to melt and liow toward said surface leaving said particles of zinc oxide protruding from said coating material and leaving only a thin ililm of coating material on the portions of said particles protruding from said coating material to provide developed areas on said surface which are incapable of retaining electrostatic charge; (5) repeating the procedures of steps 1), (2), (3) and (4) employing therein a blue developer powder consisting essentially of a zinc oxide and a coating material substantially the same as the Zinc oxide and coating material of said yellow developer powder and including a minor proportion of a blue coloring agent to produce thereby a blue developer powder image aiiixed to said photoconductive surface in areas thereon not covered by said yellow developer powder; (i6) repeating the procedures of steps (l), (2) and (3) employing therein a red developer powder to produce a red developer powder image in areas on said photoconductive surface not covered by either said yellow or said blue `developer powders; (7) fixing said red developer powder to said photoconductive surface.

5. The method of claim 4 wherein said red developer powder consists essentially of a zinc oxide and a coating material substantially the same as the zinc oxide and coating material of said yellow developer powder and including a minor proportion of a red coloring agent, said red developer powder being aiiixed to said photoconductive Surface in accordance with the procedure of step (4); and including the additional steps of: (8) repeating the procedures of steps (l), (2) and (3) employing therein a black developer powder to produce a` black developer powder image in areas on said photoconductive surface not covered by any of said yellow, blue and red developer powders; and (9) fixing said black developer powder to said photoconductive surface.

References Cited in the tile of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,735,785 Greig Feb. 2l, 1956 2,808,328 Jacob Oct. 1, 1957 2,907,674 Metcalfe et al. Oct. 6, 1959 2,940,847 Kaprelian June 14, 1960 

1. THE METHOD OF ELECTROSTATIC PRINTING COMPRISING THE STEPS OF: ELECTROPHOTOGRAPHICALLY PRODUCING A FIRST ELECTROSTATIC IMAGE ON A PHOTOCONDUCTIVE INSULATING SURFACE; APPLYING TO SAID ELECTROSTATIC IMAGE A FIRST FUSIBLE DEVELOPER POWDER HAVING A CHARACTERISTIC COLOR AND CONSISTING ESSENTIALLY OF PARTICLES OF SEMICONDUCTIVE ZINC OXIDE HAVING A COATING THEREON OF A THERMOPLASTIC, ELECTROSCOPIC MATERIAL HAVING A MELTING POINT SUBSTANTIALLY WITHIN THE RANGE OF FROM 90*C. TO 250*C. AND VISCOSITY NOT IN EXCESS OF 10,000 CENTIPOISES AT A TEMPERATURE SLIGHTLY ABOVE THE MELTING POINT OF SAID MATERIAL, SAID DEVELOPER POWDER WHEN FUSED, HAVING A BULK RESISTIVITY OF UP TO 1012 OHMCM; APPLYING HEAT TO SAID DEVELOPER POWDER ON SAID INSULATING SURFACE TO CAUSE SAID COATING TO MELT AND FLOW TOWARD SAID INSULATING SURFACE LEAVING SAID PARTICLES OF ZINC OXIDE PROTRUDING FROM SAID COATING MATERIAL, AND LEAVING ONLY A THIN FILM OF COATING MATERIAL ON THE PORTIONS OF SAID PARTICLES PROTRUDING FROM SAID COATING MATERIAL TO PROVIDE DEVELOPED AREAS ON SAID SURFACE WHICH ARE INCAPABLE OF RETAINING ELECTROSTATIC CHARGE; ELECTROPHOTOGRAPHICALLY PRODUCING A SECOND ELECTROSTATIC IMAGE ON SAID INSULATING SURFACE BEARING SAID FIRST DEVELOPER POWDER; AND APPLYING TO SAID SECOND ELECTROSTATIC IMAGE A SECOND ELECTROSCOPIC DEVELOPER POWDER HAVING A CHARACTERISTIC COLOR DIFFERENT FROM THAT OF SAID FIRST DEVELOPER POWDER WHEREBY SAID DEVELOPER POWDER IS PREVENTED FROM DEPOSITING ON SAID INSULATING SURFACE IN AREAS COVERED BY SAID FIRST DEVELOPER POWDER. 